Recently, high temperature operations with low humidification levels for polymer electrolyte fuel cells (PEFCs) have received a great deal of attention, because they have potential for solving the problem of CO poisoning, enhance electrochemical reactions, simplify the design of water and thermal subsystems and reduce the cost of PEFCs. Despite the fact that Nafion-based membranes are the best performing, commercially available polymer electrolytes, the development of other polymer membranes and ionomer binders having excellent durability even at high temperature conditions is in urgent need for alternative proton conductive materials due to the instability of Nafion-based polymers at elevated temperatures. For example, it is expected that hydrocarbon-based polymers substitute for such materials.
However, extensive studies have reported that a hydrocarbon-based ionomer binder is not appropriate for use in catalyst layers because of the strong adsorption of the aromatic rings and sulfonate groups (—SO3H) of the hydrocarbon-based ionomer on the Pt catalyst surface, which leads to significant inhibition of the oxygen reduction reaction (ORR). Furthermore, when the hydrocarbon-based binder is used in a catalyst layer under a high-relative humidity condition, the low diffusion coefficient of water and the high swelling property and low gas permeability of the hydrocarbon-based ionomer binder in the catalyst layer have detrimental effects on the efficient oxygen mass transport. Although hydrocarbon polymers having polar groups have high water uptakes over a wide temperature range, the absorbed water is restricted to the polar groups of the polymer chains. Thus, relative humidity conditions have a greater effect on the water retention and proton conductivity of the hydrocarbon ionomer as compared to the Nafion ionomer. This restricts the application of hydrocarbon ionomers to catalyst layer binders under low humidity conditions. Under these circumstances, it is expected that Nafion ionomers rather than hydrocarbon-based ionomers are used as binders in PEFCs even though they have low durability under high-temperature conditions. In addition, Nafion may be affected by reduced ORR quality of Pt catalysts due to the specific adsorption phenomenon caused by sulfonate groups. Thus, it is required to solve this problem. This is because enhancing ORR quality of Pt catalysts by mitigating the specific adsorption significantly reduces the amount of Pt required to be supported on a carrier during the manufacture of a membrane electrode assembly (MEA) for fuel cells.
However, the proton conductivity of Nafion ionomers is commonly altered dramatically under low humidity conditions. One promising strategy to increase the water content in the polymer matrix is to incorporate nanometer-sized particles of hygroscopic metal oxides into the cathode and/or anode catalyst layers. The incorporation of hygroscopic metal oxides is usually conducted by dispersing the nanoparticles synthesized by an ex situ sol-gel process. However, a notable issue is that hygroscopic oxide particles synthesized by ex situ sol-gel reactions may be easily lost or may significantly aggregate in a catalyst layer since the oxide particles are just simply mixed and not immobilized in these electrodes during the operation of a fuel cell. This results in degradation of the quality of a fuel cell during its operation. Furthermore, the addition of excessive oxide particles may cause a significant increase in the interfacial contact resistance between the catalyst layer and the polymer membrane, resulting in significant degradation of the quality of a fuel cell. When a large amount of oxide particles are incorporated to a catalyst layer, they may deteriorate the charge transfer dynamics (i.e., proton and electron transfer) of the electrodes due to the insulating properties of the oxide particles. To solve this, it is required that such hygroscopic metal oxide particles having a nanometer-scaled size are dispersed uniformly without aggregation among themselves.